This invention pertains to methods and apparatus for cardiac rhythm management. In particular, the invention relates to methods and apparatus for providing ventricular resynchronization therapy.
Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any other functions it may perform such as cardioversion or defibrillation.)
The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart""s inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate.
Pacing therapy can also be used in the treatment of congestive heart failure (CHF), which is a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet metabolic demand. CHF can be due to a variety of etiologies. with that due to ischemic heart disease being the most common. Some CHF patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. It has also been shown, however, that some CHF patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of right and left ventricular contractions with electrical stimulation. Cardiac rhythm management devices have therefore been developed which provide electrical stimulation to one or both ventricles in an attempt to improve the coordination of ventricular contractions, termed ventricular resynchronization therapy. Such electrical stimulation will be referred to herein as xe2x80x9cpacingxe2x80x9d even if the stimulation is not delivered so as to enforce a particular heart rate.
Cardiac resynchronization pacing modes involve the delivery of paces to a pacing site based upon events occurring at another site. In such modes, one chamber is designated the rate chamber or rate site, and one or more pacing sites in the same or contralateral chamber are designated synchronized sites. The synchronized sites are paced upon expiration of escape intervals which are reset by senses or paces occurring at the rate site. The present invention is concerned with the effect of transitions between pacing modes in which a pacing site is paced at different pacing instants with respect to expiration of a programmed escape interval that is reset by a sense or pace from another site. During the cardiac cycle in which the pacing mode transition takes place, a synchronized site may receive a pace at an abnormally short pacing interval if it is paced at an earlier pacing instant with respect to the escape interval expiration in the mode that is switched to, where a pacing interval is the time between successive paces delivered to the site in the absence of intrinsic activity. This is because the pacing mode itself does not directly control the interval at which a pace occurs until after the transition is completed. Such a shortened pacing interval may have adverse consequences. In accordance with the present invention, if a pacing interval for a pacing site would be below a specified minimum pacing interval value during a pacing mode transition, the pace may be inhibited or the pacing interval lengthened for the cardiac cycle in which the transition takes place.
Exemplary embodiments of the invention are applied to situations where the left or both ventricles are paced upon expiration of escape intervals reset by right ventricular senses or paces, and where there: is a transition to or from a biventricular pacing mode with a ventricular offset interval. In those situations, the left ventricle is paced in accordance with a first pacing mode such that a pace is delivered at a specified pacing instant defined with respect to expiration of a ventricular escape interval, and a transition is made to a second pacing mode in which the left ventricle is paced at an earlier specified pacing instant defined with respect to expiration of a ventricular escape interval than in the first pacing mode. In accordance with the invention, a left ventricular pace is delayed during the cardiac cycle in which the transition occurs if the left ventricular pacing instant occurs at an interval less than a specified minimum pacing interval. In another embodiment, the pace is inhibited for that cardiac cycle if the pacing interval would be below the specified minimum value.